I have been looking into this myself recently, and what I have learnt is summarised below.
There are two sets of options with each having a range of 3 main versions, from Datacolor (the Spyder5 Express/Pro/Elite) and from X-rite (Colormunki Smile/ Display and i1 Display Pro). Prices are roughly ~£70, ~£120 and ~£170.
The Spyder5 hardware is pretty much identical for all three versions, the main dfffrence is how flexible the software is,.
The Colourmunki Smile hardware seems to be basic. The other two options from X-rite use similar hardware (although the i1 Display Pro calibrates a little quicker than the Colormunki Display), again the difference between these two is how flexible the software is.
Both systems can be used with third party software (including freeware), so you can get all of the capability by using the cheaper options.
The hardware for the Colormunki Display / i1 Display Pro seems to be a little better than the Spyder5 hardware, but these are similar in price to the Spyder5 Pro and Elite (and more expensive than the Spyder5Express. The best lowest price option therefore seems to be the Spyder5Express, however if you want better performance then the Colormunki Display is a better choice. I have just ordered this, but haven't received it yet.
Further info on the options can be found here
http://www.avsforum.com/forum/139-display-calibration/1997586-new-spyder-5-vs-i1display-pro-colormunki-display.html